Nickel-base superalloys are used in the portions of aircraft gas turbine engines which have the most demanding performance requirements and are subjected to the most adverse environmental conditions. Cast nickel-base superalloys are employed, for example, as turbine blades and turbine vanes. Wrought nickel-base superalloys are employed, for example, as rotor disks and shafts. The present invention is concerned with the wrought nickel-base superalloys.
The wrought nickel-base superalloys are initially supplied as cast ingots, which are cast from the melt, or as consolidated-powder billets, which are consolidated from powders. The consolidated-powder billets are preferred as the starting material for many applications because they have a uniform, well-controlled initial microstructure and a fine grain size. In either case, the billet is reduced in size in a series of steps by metal working procedures such as forging or extrusion, and is thereafter machined. In one form of forging, the billet is placed between two forging dies in a forging press. The forging dies are forced together by the forging press to reduce the thickness of the billet.
The selection of the forging conditions depends upon several factors, including the properties and metallurgical characteristics of the nickel-base superalloy and the properties of the materials of the forging dies. The forging dies must be sufficiently strong to deform the material being forged, and the forged superalloy must exhibit the required properties at the completion of the forging operation.
At the present time, nickel-base superalloys, such as Rene™ 88DT and ME3 alloys, are isothermally forged at or above a temperature of about 1900° F. using TZM molybdenum dies. This combination of the superalloy being forged and the die material allows the forging to be performed, and the superalloy has the required properties at the completion of the forging. However, this combination of temperature, the superalloy being forged, and the die material requires that the forging be performed in vacuum or in an inert-gas atmosphere. The requirement of a vacuum or an inert-gas atmosphere greatly increases the complexity and cost of the forging process.
There is a need for an improved approach to the forging of nickel-base superalloys that achieves the required properties and also reduces the forging cost. The present invention fulfills this need, and further provides related advantages.